1. Field of the Invention
The present invention relates to communication systems, and more particularly to data rate optimization in a communication system.
2. Background
Various technologies have been developed, such as Digital Subscriber Line (DSL) technology, to provide high data rates over ordinary twisted pair lines. Numerous various standards are currently used for transferring data over twisted pair lines.
One such standard is V.90 (with a later version called V.92) published by the International Telecommunication Union (“ITU”), incorporated herein by reference in its entirety. The V.90/V.92 standard defines the operation of a digital and analog modem for data transmission in a digital communication system. The V.90/V.92 standard uses M-ary Pulse Amplitude Modulation (“PAM”) that samples signals at fixed intervals. Typically, a binary input stream is sub-divided into block of k bits called symbols and each symbol is represented by a pulse amplitude value with M possible levels where M is the number of points in a signal constellation or constellation size.
In the V.90/V.92 standard, the symbol rate is restricted by G.711 PCM codec analog/digital (A/D) sampling rate, which is typically 8 kHz. In order to get a high bit rate (or data rate) a large constellation size per symbol is required. In PAM systems, every extra bit per symbol requires more power than in Quadrature Amplitude Modulation (QAM) systems. For example to double the data rate without increasing bandwidth (symbol rate), the constellation size increases from M to M2 and energy per bit (Eb) will increase from Eb to Eb=(M2+1)Eb/2
Another standard in the DSL arena is “G.shdsl” or G.992.1, published by ITU and incorporated herein by reference in its entirety. G.shdsl digitally uses Trellis Coded Pulse Amplitude Modulation (TC-PAM) scheme with lower band of frequencies to achieve high performance range while maintaining the ability to symmetrically transmit voice or data.
In G.shdsl, the constellation size is fixed (16-TCM-PAM) and variable bit rates are achieved by varying the symbol rate. For a flat to mild amplitude distortion, if symbol rate is unlimited, the bit error probability is given by Pe, where
      P    e    =      0.5    ⁢          Q      ⁡              (                                            2              ⁢                              E                b                                                    N              0                                      )            which is independent of data and the consumed bandwidth, and where
      Q    ⁡          (      x      )        =            1                        2          ⁢          π                      ⁢                  ∫        x        ∞            ⁢                        ⅇ                                    -                              t                2                                      /            2                          ⁢                  ⅆ          t                    is the Q-function. However, signal power must increase linearly with data rates to maintain constant energy per bit (Eb)
For channels such as DSL that are distorted or attenuated as a function of frequency, the trade off between symbol rate and increasing the number of levels (i.e. increasing the constellation size), must be optimized in order to fully utilize channel capacity. In a DSL environment, cross talk from adjacent pairs in the same bundle increases with frequency. Conventional systems (i.e. ITU V.90/V.92 or G.992.1) do not allow the simultaneous choice of optimizing constellation size and symbol rates to achieve maximum throughput in data rate(s).
Therefore, there is a need for a system and method for maximizing data rate by selecting proper constellation size with a matching symbol rate for available bandwidth and average power constraint, such that each channel determines the optimal setting for constellation size and symbol rate.